Conventional apparatus for bulge testing an article comprises a pressure vessel arranged to define a chamber and in use the article being arranged over an aperture in the pressure vessel to seal the pressure vessel. A source of pressurised fluid is arranged to supply fluid to the chamber to cause the article to deform or bulge. Means to measure the material properties of the article are provided by measuring the pressure of the fluid supplied to the chamber and the deflection of the article.
U.S. Pat. No. 4,735,092 provides a micrometer to measure the vertical displacement of the article when the article has been deformed and the tensile strength of the article is calculated.
WO02/47372A provides a Michelson interferometer that generates an interference pattern having fringes and nodes that move when the article has been deformed and the material properties of the article are calculated.